Easily the most widely used video display in the United States is based on a video frame having a 525 line raster. Such displays are found in almost every American home and business in the form of televisions and VCR monitors. However, there exist certain video applications that use a display based on a video frame having more than 525 lines per frame in order to achieve greater resolution.
For example, military aircraft use cockpit video display formats that are based on both 675 and 875 lines per frame. During testing of the aircraft, it is desirable to transmit the information displayed in the aircraft to a ground station for real-time evaluation. Since the ground station locations typically use conventional 525 line video displays, it is desirable to convert the aircraft's video frame to the standard 525 line video frame. In other applications, such as medical instrumentation, high resolution video is often used. However, review of the video cannot be performed without special (and usually expensive) monitors. Accordingly, there exists a similar conversion need to allow the use of conventional monitors.
Prior art scan rate converters, as they are called, are available to generate a 525 line video frame from a video frame characterized by a greater number of lines. Their design approach is to digitally store the video signal and to retrieve whole lines of the stored data to generate the output video signal based on 525 lines per frame. Accordingly, entire lines of the stored signal are randomly skipped or in some cases, blocks of lines such as the bottom of the picture are skipped in order to generate the 525 line video frame. Typically, this results in discarding complete sections of the original video frame which may contain critical test data. It would, however, be desirable if the conversion process merely decreased the resolution of the video frame to the resolution normally associated with 525 line video.